Harsh environments are common in many applications, including conversion of fossil fuels. Coal gasification is one example of a process with extreme temperatures, chemical aggressiveness, mechanical abrasion, and pressure conditions. At the same time, coal gasification is a promising technology for efficient energy production and a component of the U.S. Department of Energy's (DOE's) Vision 21 program. Several technological challenges can impact the reliability and economics of coal gasification. One such challenge is the degradation of gasifier refractory by chemical and physical mechanisms, as illustrated in FIGS. 1A-1F.
For example, a new refractory liner of a refractory (100) used in slagging gasifiers may contain internal cracks (110) from pressing and firing, as illustrated in FIG. 1A. During a preheat process, the refractory may experience pinch spalling (120A and 120B) due to hoop stresses, as illustrated in FIG. 1B. With gasification, infiltration and/or corrosion (130) can occur to the refractory due to extreme temperatures, chemical aggressiveness, mechanical abrasion, and pressure conditions, as illustrated in FIG. 1C. Molten slag can infiltrate the cracks and pores on the hot face of the refractory during molten slag infiltration, and surface corrosion due to slag infiltration can begin. Horizontal crack formation (140A-140C) in the refractory can occur due to thermal cycling, stress accumulation, and creep, as illustrated in FIG. 1D. When cracks join, internal void formation (150) in the refractory can occur, spalling (peeling) can begin, creep can occur on slag penetrated hot face areas, and hot face corrosion can continue, as illustrated in FIG. 1E. During the renewed cycle material can break off on the hot face (160) of the refractory, as illustrated in FIG. 1F. The cycle repeats with infiltration and/or corrosion, gradually wearing away the refractory lining. As a result of the stresses on the refractory, the refractory may be replaced as often as every 10-18 months at a substantial cost and loss of production. In extreme cases, the refractory must be replaced as often as every 90 days in high-wear zones of coal gasifiers.
An improved refractory system and sensors for monitoring the refractory system can be beneficial in improving the gasification process and in reducing the costs associated with gasification. Sensors that provide direct or indirect measurements of the refractory temperature, thickness, and changes in material properties can be used to monitor and manage the refractory.
Direct measurement sensors capable of reliably performing in the harsh environment over an extended period of operation to monitor refractory degradation have not been available. Furthermore, continuous measurements of operating conditions (such as temperatures, pressures, and concentrations) in the reaction zone can be difficult to obtain and may not be utilized in practice because even the most hardened conventional sensors can quickly lose functionality in a harsh gasification environment.
The measurements used to monitor gasification and manage the refractory have been obtained using direct or indirect measurements. The direct measurement approach attempts to develop hardened sensors that can withstand harsh gasification environment for a prolonged period of time, such as thermocouples intended to provide physical and chemical resistance to attacks by most gasifier slags. Such direct measurement sensors can use heavy sheathing to improve service life of insertion sensors, but also can make such devices less sensitive to dynamic changes in temperatures, which can be important in the refractory life management since rapid temperature variations can introduce thermal stresses.
In the informative indirect (or secondary) measurement approach, temperatures, pressures, and compositions of streams into and out of a gasifier can be used with appropriate models to infer (or estimate) in real time otherwise inaccessible operating parameters inside the reaction zone and the state of the refractory itself. The combination of secondary measurements, the models that use them as the inputs, and the methods for the model-based estimation of gasification parameters of primary interest based on secondary measurements form the core of the second approach to the refractory management. Inferential measurements have been used in many process industries.
However, two limitations can exist with inferential measurements. First, the quality of inferences can depend on modeling errors and uncertainties, un-modeled changes to the process itself (e.g., due to wear and aging), the input information, and unknown process disturbances. Second, the measurement accuracy, sensitivity, and response time of inferential measurement can compare poorly with the corresponding characteristics of the direct measurements.